Pathways to the Abyss

It’s Not Just Mud

Dr. Amanda Demopoulos is a Research Ecologist for the United States Geological Survey. Her interest in marine ecology started at the coast in New Jersey, where she spent her summer vacations playing at the water’s edge, digging in the sand and looking for animals. An 8th grade biology class that covered echinoderms and corals sealed the deal.

As an undergraduate Dr. Demopoulos studied biological, chemical, and physical oceanography at the University of Washington. During her graduate studies at the University of Hawaii-Manoa she studied benthic ecology of the deep sea and coastal mangroves; mangroves are an invasive species in Hawaii, having been introduced there over 100 years ago. After a post-doctoral fellowship at Scripps Institution of Oceanography, she joined the USGS Southeast Ecological Science Center in Gainesville, FL. Her work examines benthic invertebrate community structure and function, including food webs, and impacts of natural and anthropogenic disturbance on benthic ecosystem health.

The benthic mooring will collect samples of marine snow for a year, one month at at time, and then be retrieved from the bottom of the canyon. Dr. Gerard Duineveld and Stephen Wallace, Able Seaman, look on as the benthic mooring is launched. Photo by Eric Hanneman.

On the Deepwater Canyons Expedition, Amanda is back to her roots, working nights with the Box Corer crew. In the sediment samples they bring up from the bottom of the canyon, she is looking at the food quality of deep sea deposits for benthic invertebrates using carbon and nitrogen stable isotopes. She is one of several scientists on the boat using isotopes to study food webs and the environment of the deep ocean.

Drs. Kellogg and Demopoulos prepare smaller cores from the Box Corer for further study. Photo by Eric Hanneman.

There is a constant rain of plankton, animal waste and other debris entering the canyon known as marine snow. The marine snow can take two weeks to reach the bottom of the canyon, and along the way down it is processed and reprocessed by many animals. Many of these sediments are not immediately used as a food source, but instead settle on the bottom. Then ocean currents can resuspend them, bringing them back up the canyon or moving them further out onto the abyssal plain.

You have got to get dirty sometimes to get good data. Photo by Eric Hanneman.

Ecologists such as Dr. Demopoulos use the tools of biogeochemistry to study the nutrients animals incorporate into their bodies. The four main items are particulate organic matter and carbon (POM and POC) and dissolved organic matter and carbon (DOC and DOM). From the water column, measurements of isotopes of carbon and nitrogen (DOC and DOM) are taken, and from the particles found in the water column (POC and POM) other isotopes including Mg, I, Sr, Ca are measured. As the isotopes decay, the time the nutrients have been incorporated into the food web and where they are in food chain can be determined.

Another one of the goals of the expedition is to use deep sea corals to address changes in circulation of ocean currents and the nutrients they carry. Dr. Brendan Roark examines the age and growth rates of deep sea corals using radioisotope dating. The calcitic spicules that support Paragorgia arborea are derived from calcium that is in the water, and from the marine snow. Stony hexacorals can be dated like trees by measuring their growth rings, which is known as sclerochronology dating. Using these rings and another set of isotopes to confirm the age of the coral, a further set of isotopes can then be used to measure the temperature of the ocean when the coral was forming. Using pink coral in Hawaii, researchers have shown that the Sr/Ca ratio varies with temperature, going back 120 years. Hawaii black coral is up to 4000 years old, Gulf of Mexico black coral skeletons have a record of the last 2,000 years. By calculating changes in ocean temperature, this can be used to infer changes in ocean circulation, since surface and deep ocean currents have different temperatures.

This sediment core was retrieved from close proximity to a cold seep, a methane vent in the canyon. Some how this worm calls this unusual environment home. Photo by Eric Hanneman.

Using the tools of biogeochemistry can open windows into the past. The recent past is revealed by the benthic infauna found in sediments and their relationship to the food web and the water column. The ancient past is revealed by the ocean chemistry recorded in the skeletons of corals. These are only some of the new frontiers that the scientists of the Deepwater Canyons Expedition are exploring.